Magmatic-hydrothermal origin for Carlin-type Au deposits: Evidences from in-situ S-Pb isotopic compositions of sulfides, the Chang’an Au deposit, southern Ailaoshan tectonic zone

Abstract. Monoclinic epidote is a low-µ (µ = 283U / 204Pb) mineral occurring in a variety of geological environments, participating in many metamorphic reactions and stable throughout a wide range of pressure–temperature conditions. Despite containing fair amounts of U, its use as a U–Pb geochronometer has been hindered by the commonly high contents of initial Pb with isotopic compositions that cannot be assumed a priori. We present U–Pb geochronology of hydrothermal-vein epidote spanning a wide range of Pb (3.9–190 µg g−1), Th (0.009–38 µg g−1) and U (2.6–530 µg g−1) contents and with µ values between 7–510 from the Albula area (eastern Swiss Alps), from the Grimsel area (central Swiss Alps) and from the Heyuan fault (Guangdong province, China). The investigated epidote samples show appreciable fractions of initial Pb that vary to different extents. A protocol has been developed for in situ U–Pb dating of epidote by spot-analysis laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) with a magmatic allanite as primary reference material. The suitability of the protocol and the reliability of the measured isotopic ratios have been ascertained by independent measurements of 238U / 206Pb and 207Pb / 206Pb ratios respectively by quadrupole and multicollector ICP–MS applied to epidote micro-separates digested and diluted in acids. For age calculation, we used the Tera–Wasserburg (207Pb / 206Pb–238U / 206Pb) diagram, which does not require corrections for initial Pb and provides the initial 207Pb / 206Pb ratio if all intra-sample analyses are co-genetic. Petrographic and microstructural data indicate that the calculated ages date the crystallization of vein epidote from a hydrothermal fluid and that the U–Pb system was not reset to younger ages by later events. Vein epidote from the Albula area formed in the Paleocene (62.7 ± 3.0 Ma) and is related to Alpine greenschist-facies metamorphism. The Miocene (19.1 ± 4.0 Ma and 16.9 ± 3.7 Ma) epidote veins from the Grimsel area formed during the Handegg phase (22–17 Ma) of the Alpine evolution of the Aar Massif. Identical initial 207Pb / 206Pb ratios reveal homogeneity in Pb isotopic compositions of the fluid across ca. 200 m. Vein epidote from the Heyuan fault is Cretaceous in age (108.1 ± 8.4 Ma) and formed during the early movements of the fault. In situ U–Pb analyses of epidote returned reliable ages of otherwise undatable epidote-quartz veins. The Tera–Wasserburg approach has proven pivotal for in situ U–Pb dating of epidote and the decisive aspect for low age uncertainties is the variability in intra-sample initial Pb fractions.

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Supplemental Material: Rifting and subduction records of the Paleo–Tethys in North Laos: Constraints from Late Paleozoic mafic and plagiogranitic magmatism along the Song Ma tectonic zone

10.1130/gsab.s.12252230 ◽

2020 ◽

Author(s):

Yuzhi Zhang

Keyword(s):

Late Paleozoic ◽

Tectonic Zone ◽

Analytical Results ◽

Trace Elemental ◽

Host Rocks

Table S1: Major, trace elemental and Sr-Nd isotopic analytical results for the mafic and plagiogranitic rocks along the Song Ma tectonic zone in North Laos; Table S2: SIMS and LA-ICPMS zircon U-Pb dating results for the mafic, plagiogranitic and associated granitic host rocks along the Song Ma tectonic zone in North Laos; Table S3: Zircon in-situ Hf and O isotopic analytical results for the mafic and plagiogranitic rocks along the Song Ma tectonic zone in North Laos.

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Mineral chemistry, P-T pseudosection and in-situ U-Th-Pbtotal monazite geochronology of Banded Iron Formation from Bundelkhand craton North-Central India, and its geodynamic significance

10.5194/egusphere-egu21-9812 ◽

2021 ◽

Author(s):

Mohd Baqar Raza ◽

Pritam Nasipuri ◽

Hifzurrahman

Keyword(s):

Central India ◽

Mineral Chemistry ◽

Iron Formation ◽

Age Group ◽

Banded Iron Formation ◽

Tectonic Zone ◽

Third Age ◽

The Third ◽

Hydrothermal Activities

The Banded Iron Formation (BIF) in Bundelkhand craton (BuC) occurred as supracrustals associated with TTG&#8217;s, amphibolites, calcsilicate rocks, and quartzite within the east-west trending Bundelkhand tectonic zone (BTZ). The BIFs near Mauranipur do not show any prominent iron-rich and silica-rich layer band and are composed of garnet, amphibole, quartz, and magnetite. The volumetrically dominant monoclinic-amphiboles are grunerite in composition. XMg of grunerite varies between 0.39-0.37. The garnets are Mn-rich, the XSpss of garnet ranges from 0.26-0.20, XPyp and XGrs vary between 0.10-0.06 and 0.07-0.05, respectively. P-T pseudosection analysis indicates that by destabilizing iron-silicate hydroxide phases through a series of dehydration and decarbonation reactions, amphibole and garnet stabilized in BIF at temperature 400-450&#176;C and pressure 0.1-0.2 GPa.Massive type BIFs have monazite grains that vary from 10 to 50 &#181;m in size, yield three distinct U-Th-Pbtotal age clusters. 10-20 &#181;m sized monazite grains yield the oldest age, 3098&#177;95 Ma. 2478&#177;37 Ma average age is obtained from the second group, which is relatively larger and volumetrically predominant. The third age group of Monaiztes gives an age of 2088&#177;110 Ma. ~3100 Ma monazite suggests the older supracrustal rocks of Bundelkhand craton, similar to those obtained from Singhbhum and the Dharwar craton. The 2478&#177;37 Ma age is constrained as the timing of metamorphism and stabilization of BuC. The third age group, 2088&#177;110 Ma probably associated with renewed hydrothermal activities, leading to rifting and emplacement of mafic dykes in BuC.

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Constraints of fluid inclusions and in-situ S-Pb isotopic compositions on the origin of the North Kostobe sediment-hosted gold deposit, eastern Kazakhstan

Ore Geology Reviews ◽

10.1016/j.oregeorev.2016.10.004 ◽

2017 ◽

Vol 81 ◽

pp. 256-269 ◽

Cited By ~ 8

Author(s):

Kam-Hung Wong ◽

Mei-Fu Zhou ◽

Wei Terry Chen ◽

Hugh O'Brien ◽

Yann Lahaye ◽

...

Keyword(s):

Gold Deposit ◽

Fluid Inclusions ◽

Isotopic Compositions ◽

The North ◽

Eastern Kazakhstan

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The chlorine-isotopic composition of lunar KREEP from magnesian-suite troctolite 76535

American Mineralogist ◽

10.2138/am-2020-7467 ◽

2020 ◽

Vol 105 (8) ◽

pp. 1270-1274

Author(s):

Francis M. McCubbin ◽

Jessica J. Barnes

Keyword(s):

Isotopic Composition ◽

Melt Inclusions ◽

Melt Inclusion ◽

Isotopic Fractionation ◽

Magma Ocean ◽

Geochemical Composition ◽

Isotopic Compositions ◽

Stable Isotopic ◽

Lunar Samples

Abstract We conducted in situ Cl isotopic measurements of apatite within intercumulus regions and within a holocrystalline olivine-hosted melt inclusion in magnesian-suite troctolite 76535 from Apollo 17. These data were collected to place constraints on the Cl-isotopic composition of the last liquid to crystallize from the lunar magma ocean (i.e., urKREEP, named after its enrichments in incompatible lithophile trace elements like potassium, rare earth elements, and phosphorus). The apatite in the olivine-hosted melt inclusion and within the intercumulus regions of the sample yielded Cl-isotopic compositions of 28.3 ± 0.9‰ (2σ) and 30.3 ± 1.1‰ (2σ), respectively. The concordance of these values from both textural regimes we analyzed indicates that the Cl-isotopic composition of apatites in 76535 likely represents the Cl-isotopic composition of the KREEP-rich magnesian-suite magmas. Based on the age of 76535, these results imply that the KREEP reservoir attained a Cl-isotopic composition of 28–30‰ by at least 4.31 Ga, consistent with the onset of Cl-isotopic fractionation at the time of lunar magma ocean crystallization or shortly thereafter. Moreover, lunar samples that yield Cl-isotopic compositions higher than the value for KREEP are likely affected by secondary processes such as impacts and/or magmatic degassing. The presence of KREEP-rich olivine-hosted melt inclusions within one of the most pristine and ancient KREEP-rich rocks from the Moon provides a new opportunity to characterize the geochemistry of KREEP. In particular, a broader analysis of stable isotopic compositions of highly and moderately volatile elements could provide an unprecedented advancement in our characterization of the geochemical composition of the KREEP reservoir and of volatile-depletion processes during magma ocean crystallization, more broadly.

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Isotopic analyses of clinopyroxene in mantle xenoliths demonstrate the effects of kimberlite melt metasomatism upon the lithospheric mantle

10.5194/egusphere-egu2020-5331 ◽

2020 ◽

Author(s):

Angus Fitzpayne ◽

Andrea Giuliani ◽

Janet Hergt ◽

Jon Woodhead ◽

Roland Maas

Keyword(s):

Trace Element ◽

Lithospheric Mantle ◽

Mantle Metasomatism ◽

Mantle Xenoliths ◽

Isotopic Compositions ◽

Isotopic Analyses ◽

Cretaceous Magmatism

As clinopyroxene is the main host of most lithophile elements in the lithospheric mantle, the trace element and radiogenic isotope systematics of this mineral have frequently been used to characterise mantle metasomatic processes. To further our understanding of mantle metasomatism, both solution-mode Sr-Nd-Hf-Pb and in situ trace element and Sr isotopic data have been acquired for clinopyroxene grains from a suite of peridotite (lherzolites and wehrlites), MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside), and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks from the Kimberley kimberlites (South Africa). The studied mantle samples can be divided into two groups on the basis of their clinopyroxene trace element compositions, and this subdivision is reinforced by their isotopic ratios. Type 1 clinopyroxene, which comprises PIC, wehrlite, and some sheared lherzolite samples, is characterised by low Sr (~100&#8211;200 ppm) and LREE concentrations, moderate HFSE contents (e.g., ~40&#8211;75 ppm Zr; La/Zr < 0.04), and restricted isotopic compositions (e.g., 87Sr/86Sri = 0.70369&#8211;0.70383; &#949;Ndi = +3.1 to +3.6) resembling those of their host kimberlite magmas. Available trace element partition coefficients can be used to show that Type 1 clinopyroxenes are close to equilibrium with kimberlite melt compositions, supporting a genetic link between kimberlites and these metasomatised lithologies. Thermobarometric estimates for Type 1 samples indicate equilibration depths of 135&#8211;155 km within the lithosphere, thus showing that kimberlite melt metasomatism is prevalent in the deeper part of the lithosphere beneath Kimberley. In contrast, Type 2 clinopyroxenes occur in MARID rocks and coarse granular lherzolites, which derive from shallower depths (<130 km), and have higher Sr (~350&#8211;1000 ppm) and LREE contents, corresponding to higher La/Zr of >~0.05. The isotopic compositions of Type 2 clinopyroxenes are more variable and extend from compositions resembling the &#8220;enriched mantle&#8221; towards those of Type 1 rocks (e.g., &#949;Ndi = -12.7 to -4.4). To constrain the source of these variations, in situ Sr isotope analyses of clinopyroxene were undertaken, including zoned grains in Type 2 samples. MARID and lherzolite clinopyroxene cores display generally radiogenic but variable 87Sr/86Sri values (0.70526&#8211;0.71177), which might be explained by the interaction between peridotite and melts from different enriched sources with the lithospheric mantle. In contrast, the rims of these Type 2 clinopyroxenes trend towards compositions similar to those of the host kimberlite and Type 1 clinopyroxene from PIC and wehrlites. These results are interpreted to represent clinopyroxene overgrowth during late-stage (shortly before/during entrainment) metasomatism by kimberlite magmas. Our study shows that an early, pervasive, alkaline metasomatic event caused MARID and lherzolite genesis in the lithospheric mantle beneath the Kimberley area, which was followed by kimberlite metasomatism during Cretaceous magmatism. This latter event is the time at which discrete PIC, wehrlite, and sheared lherzolite lithologies were formed, and MARID and granular lherzolites were partly modified.

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Supplemental Material: A model involving amphibolite lower crust melting and subsequent melt extraction for leucogranite generation

10.1130/gsab.s.14892747 ◽

2021 ◽

Author(s):

Liqiang Wang ◽

et. al

Keyword(s):

Trace Element ◽

Lower Crust ◽

Partition Coefficients ◽

Geochemical Modeling ◽

Magma Source ◽

Isotopic Data ◽

Melt Extraction ◽

Isotopic Compositions ◽

Age Determinations

Table S1: Isotopic data of U-Pb age determinations on zircons of the Anglonggangri biotite-muscovite and garnet-muscovite granites; Table S2: 40Ar-39Ar dating results for muscovite from the pegmatite in the Anglonggangri area; Table S3: Whole-rock major and trace element compositions of the Anglonggangri biotite-muscovite and garnet-muscovite granites; Table S4: Whole-rock Pb isotopic compositions of the Anglonggangri biotite-muscovite and garnet-muscovite granites; Table S5: Zircon in situ Lu-Hf isotopic compositions of the Anglonggangri biotite-muscovite granite; Table S6: Partition coefficients and assumed magma source compositions used in geochemical modeling; Table S7: Partition coefficients and assumed compositions used in geochemical modeling and the calculated results.

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